There are known optical systems for providing a variable field of view, such as a catadioptric optical system providing discrete wide and narrow fields of view by movement of optical elements into and out of the optical path. The movement of these elements is accomplished by complex mechanical linkages of optical components.
An example of this type of system is disclosed in U.S. Pat. No. 4,354,742 to Abel et al., based on the movement of a secondary mirror in and out of the path of a fixed objective. Another example of this approach is disclosed in European Patent Application 84110278.2, based on a secondary reflector which is pivoted by means of an actuator cable for movement from a narrow to a wide field of view position. Still another example of this arrangement is found in British Patent No. 1,423,257, where a rotatable lens is used.
The aforementioned European Patent Application also discloses an embodiment in which the secondary reflector is replaced by a material such as vanadium oxide which, when heated by an electrical circuit, changes from a transparent body to a reflector body. The vanadium oxide or any such reflective and transparent element has the disadvantage that it is neither completely transparent nor completely reflective. Due to the fact that this element is not completely transmissive in the wide field of view as disclosed in the subject application at page 6, lines 13-16, there is interference by reflection of the narrow field of view light from this element. Although it is stated that the radiant energy of reflection is very low, the interference is significant. Thus, the overall performance of the optical system is reduced.
Applications of a dual field of view system using movable lenses are also found in cameras, as disclosed in German Patent No. DE 3,444,753 and U.S. Pat. No. 4,037,939.
In stabilized platform applications, where the system is highly sensitive to the static balance, mechanical complexity of the aforementioned systems introduces unwanted disturbances.
Therefore, it would be desirable to provide a reliable dual field of view optical system which is based on a simplified optical arrangement.